The present invention relates to a crosslinked, heat resistant elastic article having elevated temperature elasticity comprising a cured, irradiated or crosslinked ethylene polymer and a method for making a crosslinked, heat resistant elastic article. In particular, the invention relates to a shaped article (e.g. film or fiber) characterized by heat resistance and improved elasticity at elevated temperatures and comprising a substantially cured, irradiated, or crosslinked homogeneously branched ethylene polymer. The improved elastic article of the present invention is particularly suitable for use in applications where good elasticity must be maintained at elevated temperatures such as, for example, personal hygiene items and disposable infection-control garments at body temperatures of about 100xc2x0 F. (38xc2x0 C.).
Materials with excellent stretchability and elasticity are needed to manufacture a variety of disposal and durable articles such as, for example, incontinence pads, disposable diapers, training pants, sport apparel and furniture upholstery. Stretchability and elasticity are performance attributes which function to effectuate a closely conforming fit to the body of the wearer or to the frame of the item. It is desirable to maintain the conforming fit during repeated use, extensions and retractions at body temperatures. Further, for incontinence articles, stretchability and elasticity are particularly desirable to ensure comfort and provide security against unwanted leaks.
Disposable articles are typically elastic composite materials prepared from a combination of polymer film, fibers, sheets and absorbent materials as well as a combination of fabrication technologies. Whereas the fibers are prepared by well known processes such as spun bonding, melt blowing, melt spinning and continuous filament wounding techniques, the film and sheet forming processes typically involve known extrusion and coextrusion techniques, e.g., blown film, cast film, profile extrusion, injection molding, extrusion coating, and extrusion sheeting.
A material is typically characterized as elastic where it has a high percent elastic recovery (i.e., a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties, i.e., a low percent permanent set, a low stress or load at strain, and a low percent stress or load relaxation. That is, there should be (1) a low stress or load requirement to stretch the material, (2) no or low relaxing of the stress or unloading once the material is stretched, and (3) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
Lycra (spandex) is a segmented polyurethane elastic material which is known to exhibit goodelastic properties. But Lycra tends to be extremely cost prohibitive for a many of applications. Also, Lycra like natural rubbers tend to exhibit poor environmental resistance to ozone, chlorine and high temperature, especially in the presence of moisture.
Natural rubbers, as discussed by Ferdinand Rodriguez in Principles of Polymer Systems, pp. 242-43, McGraw-Hill (1982), the disclosure of which is incorporated herein by reference, generally show decreases in elongation to break with increase in degree of crosslinking. Furthermore, at high degrees of crosslinking, even tenacity at break may decrease for natural rubbers.
Elastic materials such as films, strips, coating, ribbons and sheet comprising at least one substantially linear ethylene polymer are disclosed in U.S. Pat. No. 5,472,775 to Obijeski et al., the disclosure of which is incorporated herein by reference. But U.S. Pat. No. 5,472,775 does not disclose the performance of these materials at elevated temperatures (i.e., at temperatures above room temperature).
WO 94/25647 (Knight et al.), the disclosure of which is incorporated herein by reference, discloses elastic fibers and fabrics made from homogeneously branched substantially linear ethylene polymers. The fibers are said to posses at least 50 percent recovery (i.e., less than or equal 50% permanent set) at 100 percent strain. But there is no disclosure in WO 94/25647 regarding the elasticity of these fibers at elevated temperatures, nor is there any disclosure regarding resistance to high temperatures.
U.S. Pat. No. 5,322,728 to Davey et al., the disclosure of which is incorporated herein by reference, discloses elastic fibers comprised of single site catalyzed ethylene polymers. But polymers are not cured, irradiated or crosslinked and therefore are believed to exhibit poor elevated temperature elasticity.
WO 95/29197 (Penfold et al.), the disclosure of which is incorporated herein by reference, discloses curable, silane-grafted substantially ethylene polymers which are useful for use in wire and cable coatings, weather-stripping and fibers. WO 95/29197 reports examples which include fibers comprising silane-grafted substantially ethylene polymers having densities of 0.868 g/cm3 and 0.870 g/cm3. While example fibers are shown to exhibit improved elastic recovery at elevated temperatures, there is no disclosure regarding percent stress or load relaxation performance at elevated temperatures.
U.S. Pat. No. 5,324,576 to Reed et al., the disclosure of which is incorporated herein by reference, discloses an elastic nonwoven web of microfibers of radiation crosslinked ethylene/alpha olefin copolymers, preferably having a density less than 0.9 g/cm3. The examples reported in U.S. Pat. No. 5,324,576 comprise ethylene polymers having polymer densities greater than or equal to 0.871 g/cm3 which subjected to electron beam radiation. But Reed et al. provide no disclosure regarding the elastic performance of these radiated polymers at elevated temperatures.
U.S. Pat. No. 5,525,257 to Kurtz et al., the disclosure of which is incorporated herein by reference, discloses that low levels of irradiation of less than 2 megarads of Ziegler catalyzed linear low density ethylene polymer results in improved stretchability and bubble stability without measurable gelation.
U.S. Pat. No. 4,425,393 to Benedyk et al., the disclosure of which is incorporated herein by reference, discloses low modulus fibers having diameters in the range of 0.5 to 3 mils (about 1 to about 37 denier).
Canadian Patent No. 935,598 to Hardy et al., the disclosure of which is incorporated herein by reference, discloses elastic fibers comprised of various ethylene polymers wherein the fibers are post-drawn (stretched) and crosslinked while under tension.
U.S. Pat. No. 4,957,790 to Warren, the disclosure of which is incorporated herein by reference, discloses the use of pro-rad compounds and irradiation to prepare heat-shrinkable linear low density polyethylene films having an increased orientation rate during fabrication. In the examples provided therein, Warren employs Ziegler catalyzed ethylene polymers having densities greater than or equal to 0.905 g/cm3.
In spite of various disclosures relating to elastic ethylene polymer articles, including articles comprising curable, radiated and/or crosslinked ethylene polymers, there is a present need for cost-effective elastic articles having good heat resistance and elasticity at elevated temperatures, especially at human body temperatures of about 100xc2x0 F. There is also a need for a method of making elastic articles having good elasticity at elevated temperatures. We have discovered that these and other objects can be completely met by the invention herein described.
We have discovered that elastic articles comprising a substantially cured, radiated or crosslinked ethylene polymer wherein the polymer is characterized as having a polymer density of less than 0.89 g/cm3, especially less than 0.87 g/cm3 and most especially less than or equal to 0.865 g/cm3 (or a differential scanning calorimetry (DSC) crystallinity at 23xc2x0 C. of less than 26weight percent, especially 12 weight percent, and most especially less than or equal to 8.5 weight percent). These novel articles exhibit excellent elasticity at room temperature and at elevated temperatures.
The broad aspect of the invention provides a heat resistant, shaped cured, irradiated or crosslinked article comprising an ethylene interpolymer of ethylene interpolymerized with at least one other monomer and characterized as having:
a) a polymer density of less than 0.89 g/cm3 or a DSC crystallinity at 23xc2x0 C., as determined using differential scanning calorimetry, of less than 26 weight percent before being shaped, cured, irradiated or crosslinked and
b) in meltspun fiber form, a value less than 0.75 for the expression
Abs[xcex94E/E0]+Abs[xcex94T/T0]
where xcex94E and xcex94T are taken from a stress-strain plot, as determined using an Instron tensiometer at 500 mm/minute crosshead speed and 10.2 cm gage length and from the average of four replications of five fiber samples; xcex94E is taken as the difference in percent elongation between the cured, irradiated or crosslinked polymer and the uncured, irradiated or uncrosslinked interpolymer at a tenacity of 0.4 grams/denier; E0 is taken as the percent elongation of the uncured, irradiated or uncrosslinked interpolymer at a tenacity of 0.4 grams/denier; xcex94T is taken as the difference in tenacity (in grams/denier) between the cured, irradiated or crosslinked polymer and the uncured, irradiated or uncrosslinked polymer at a percent elongation of 300 percent; T0 is taken as the tenacity (in grams/denier) of the uncured, irradiated or uncrosslinked interpolymer at a percent elongation of 300 percent; and Abs denotes absolute value.
Another aspect of the invention is a heat resistant cured, irradiated or crosslinked elastic fiber comprising ethylene interpolymerized with at least one other monomer wherein the interpolymer is characterized as having:
a) polymer density of less than 0.89 g/cm3 or a crystallinity at 23xc2x0 C., as determined using differential scanning calorimetry, of less than 26 weight percent before being shaped, cured, irradiated or crosslinked and
b) in meltspun fiber form, a value less than 0.75 for the expression
Abs[xcex94E/E0]+Abs[xcex94T/T0]
where xcex94E and xcex94T are taken from a stress-strain plot, as determined using an Instron tensiometer at 500 mm/minute crosshead speed and 10.2 cm gage length and from the average of four replications of five fiber samples; xcex94E is taken as the difference in percent elongation between the cured, irradiated or crosslinked polymer and the uncured, irradiated or uncrosslinked interpolymer at a tenacity of 0.4 grams/denier; E0 is taken as the percent elongation of the uncured, irradiated or uncrosslinked interpolymer at a tenacity of 0.4 grams/denier; xcex94T is taken as the difference in tenacity (in grams/denier) between the cured, irradiated or crosslinked polymer and the uncured, irradiated or uncrosslinked polymer at a percent elongation of 300 percent; T0 is taken as the tenacity (in grams/denier) of the uncured, irradiated or uncrosslinked interpolymer at a percent elongation of 300 percent; and Abs denotes absolute value.
A third aspect of the invention is a heat resistant shaped elastic article which comprises at least one ethylene interpolymer which has been cured, irradiated or crosslinked wherein the interpolymer comprises ethylene interpolymerized with at least one other monomer and is characterized as having:
(a) a polymer density of less than 0.87 g/cm3 before being shaped, cured, irradiated or crosslinked,
(b) a percent permanent set of less than or equal 25 at 23xc2x0 C. and 200 percent strain when measured at a 2 mil thickness using an Instron tensiometer after being shaped, cured, irradiated or crosslinked,
(c) a percent stress relaxation of less than or equal 25 at 23xc2x0 C. and 200 percent strain when measured at a 2 mil thickness using a Instron tensiometer after being shaped, cured, irradiated or crosslinked, and
(d) a percent stress relaxation of less than or equal 55 at 38xc2x0 C. and 200 percent strain when measured at a 2 mil thickness using an Instron tensiometer after.
A fourth aspect of the invention is a method of making an elastic article comprising the steps of
(a) providing an ethylene interpolymer having a density of less than 0.87 g/cm3,
(b) fabricating the article from the interpolymer, and
(c) after the fabrication, subjecting the article to heat or ionizing radiation or both.
A fifth aspect of the invention is a method of making an elastic article comprising the steps of
(a) providing an ethylene interpolymer having a density of less than 0.87 g/cm3,
(b) incorporating a pro-rad crosslink additive into the interpolymer,
(c) fabricating the article from the interpolymer, and
(d) after fabrication, subjecting the article to heat or ionizing radiation or both.
Preferably, the article is fabricated using an extrusion technique (i.e., the method consists of melting the interpolymer). Suitable extrusion techniques include, but are not limited to, fiber melt spinning, fiber melt blowing, film blowing, cast film, injection molding, or rotomolding technique. Preferably, the extrudate, filament, web or part is permitted to cool or is quenched to ambient temperature (i.e., permitted to substantially solidify) before the application of additional heating or ionizing radiation.
In a preferred embodiment of the invention, the ethylene polymer is a homogeneously branched ethylene polymer, especially a substantially linear ethylene polymer. In another preferred embodiment, the ionizing radiation is provided by electron beam irradiation.
We discovered that (unlike natural rubbers) curing, irradiation or crosslinking (increased crosslink densities) do not decrease the elongation at break or tenacity at break for homogeneously branched ethylene polymers having a polymer density of less than 0.89 g/cm3 and that articles (especially fibers) of cured, irradiated or crosslinked-homogeneously branched ethylene polymers exhibit substantially improve heat resistance.
We also discovered that there is a subset of ethylene polymers which provide completely unexpected elastic performance results when cured, radiated or crosslinked. In particular, we found for the broad range of polymer densities above and below 0 0.87 g/cm3, curing, radiation or crosslinking dramatically decrease percent permanent set performance (i.e., improve elasticity or elastic recovery) and have no substantial effect on ambient percent stress or load relaxation performance. But while tending to adversely affect (i.e., increase) or have no effect on percent stress or load relaxation at elevated temperatures for polymer having densities equal to or greater than 0.865 g/cm3, surprisingly curing, radiation and crosslinking decreases (i.e., improves) the elevated temperature percent stress or load relaxation performance of ethylene interpolymer having a polymer density less than 0.87 g/cm3 or a DSC crystallinity at 23xc2x0 C. less than 12 weight percent. That is, curing, radiating or crosslinking is an effective means for providing elastic materials and articles characterized as having excellent elevated temperature stress relaxation characteristics.
Not only is the dramatically different response to irradiation or crosslinking surprisingly in itself, these results are surprising for other reasons as well. For example, these results are surprising and unexpected because at a density of 0.87 g/cm3, ethylene polymers are already substantially amorphous. That is, a cross-over or transition in elastic performance attributable to curing, radiation or crosslinking would ordinarily be expected to relate to the amorphosity of the polymer; but according to hexane extraction data at 50xc2x0 C., determined according to the Food and Drug Administration (FDA) test method set forth under 21 37 C.F.R. xc2xa7xc2xa7177.1520 (d)(3)(ii), ethylene polymers are substantially amorphous at a density of 0.89 g/cm3 and below. Given such small differences in amorphosity or crystallinity, dramatic elasticity differences in response to irradiation or crosslinking simply would not ordinarily be expected.
Accordingly, the shaped elastic articles of present invention exhibit a unique combination of properties such as tenacity at break, elongation, elastic recovery, chlorine and aromatic/polar solvent resistance, moisture resistance, heat aging and excellent high temperature mechanical performance compared to traditional elastic materials, for example, natural rubber and spandex.